The present invention relates to measurement of a time interval between two signals, and in particular to a time interval measurement circuit requiring short measurement time suitable for requirements of high time measurement precision and high resolution.
In a conventional high-precision time interval measurement circuit, an interpolating technique is used. The interpolating technique is described in HEWLETT PACKARD 5370A UNIVERSAL TIME INTERVAL COUNTER OPERATING AND SERVICE MANUAL (1979) pp. 8-29 to 8-31. When this method is used, the time intervals between the base clock (200 MHz) within a time interval counter and the start signal as well as the stop signal are measured correctly. The time interval between the two signals is desired to be measured. By referring to FIG. 4, the method for measuring the time interval between an input signal fi 200 (start signal or stop signal) inputted to the time interval counter and the base clock will now be described. A variable clock fv 201 (having a period of 5.0195763 ns) is so generated as to begin to oscillate in synchronism with the falling edge of the input signal fi 200 and have a period nearly equal to that of the base clock fs 202 (having a period of 5 ns). Assuming that the number of pulses counted until the rising edge of the variable clock fv 201 thus generated coincides in time with the rising edge of the base clock fs 202 as N1, time interval Td between the input signal fi and the base clock fs can be represented as ##EQU1## where fv is the frequency of the variable clock, and fs is the frequency of the base clock. For improving the measurement precision, therefore, it is necessary to make ##EQU2## small. That is to say, the time interval between the period of the base clock fs and the period of the variable clock fv must be chosen so as to be small. In case of the prior art, this time interval is chosen to be approximately 20 ps. At worst, therefore, the number N.sub.1 of pulses and the time T required for the measurement are represented as ##EQU3##
In case of the above described prior art, consideration is not given to the measurement time. The prior art thus has a problem that raising the time resolution of measurement results in proportionately increased measurement time.